So is there actual any ending or just that huge monster end scene?
To get the good ending, you need a few things. Firstly, you need to be a monster. Secondly, you need to talk to one of the succubus and take what she "provides" you with. Thirdly, not sure about this one, but you probably need to pick up the tablet in one of the rooms at the start of the level.
Then, in the chamber with the big demon, activate the magician when you can. After that, DODGE, then pick up the gem and you get the ending.
P.S. What does the second lever do? The one hidden behind some boxes and shelves?